Among compound semiconductors, group III element nitrides (hereinafter, referred to as group III nitrides or group III nitride semiconductors, as the case may be) such as gallium nitride (GaN) are attracting attention as the materials for blue light or ultraviolet light-emitting semiconductor elements. Blue laser diodes (LDs) are applied to high-density optical discs or high-density displays, and blue light-emitting diodes (LEDs) are applied to displays or illumination. Ultraviolet LDs are expected to be applied to biotechnology and the like, and ultraviolet LEDs are expected to be applied as ultraviolet light sources for fluorescent lamps.
The substrates made of the group III nitride semiconductors (such as GaN) for use in LDs and LEDs are usually formed on sapphire substrates by heteroepitaxially growing group III nitride single crystals with vapor phase epitaxial growth methods. Examples of the vapor phase growth methods include the metal organic chemical vapor deposition method (MOCVD method), the hydride vapor phase epitaxy method (HVPE method) and the molecular beam epitaxy method (MBE method).
Alternatively, instead of vapor phase epitaxial growth, methods for growing crystals in liquid phase have also been investigated. The nitrogen equilibrium vapor pressure at the melting point of the single crystal of a group III nitride such as GaN or AlN is ten thousands atm or more. Accordingly, it is generally accepted that, for the purpose of growing gallium nitride in the liquid phase, known techniques require the conditions set at 1200° C. and 8000 atm (8000×1.01325×105 Pa). In contrast, recently it has been shown that the use of an alkali metal such as Na enables the synthesis of GaN at a relatively low temperature of 750° C. and a relatively low pressure of 50 atm (50×1.01325×105 Pa).
Recently, in an ammonium-containing nitrogen gas atmosphere, a mixture composed of Ga and Na was melted at 800° C. and 50 atm (50×1.01325×105 Pa), and single crystals having a maximum crystal size of 1.2 mm have been obtained by using the resulting molten liquid, with a growth time of 96 hours (for example, JP2002-293696A).
There has also been proposed a method in which after a GaN crystal layer is formed as a film on a sapphire substrate with the metal organic chemical vapor deposition (MOCVD) method, a single crystal is grown with the liquid phase epitaxy (LPE) method (for example, JP2005-263622A).
FIG. 15 shows a schematic configuration of a known production apparatus for growing a GaN crystal with the liquid phase epitaxy method. Reference numeral 100 denotes a heating growth furnace, in the interior of which an air-tight pressure-resistant heat-resistant vessel 103 is disposed. Reference numeral 104 denotes a lid of the vessel 103. Reference numeral 101 denotes a raw material gas feeder for feeding a raw material gas 109, namely, nitrogen gas, and the raw material gas feeder 101 is connected to the pressure-resistant heat-resistant vessel 103 through a connecting pipe 114. The connecting pipe 114 is equipped with a pressure regulator 102, a leak valve 106, a joint 108 and a stop valve 105. The growth furnace 100 is constructed as an electric furnace equipped with a heat insulator 111 and a heater 112, and the temperature of the growth furnace 100 is controlled with a thermocouple 113. The growth furnace 100 as a whole is capable of being swung about a horizontal shaft center A.
Inside the pressure-resistant heat-resistant vessel 103, a crucible 107 is disposed. A high-temperature raw material liquid 110 is held inside the crucible 107, and a template 201 is immersed in the raw material liquid 110. The template 201 is prepared by forming as film a semiconductor layer composed of GaN on a sapphire substrate and is used as a seed crystal. The template 201 is prepared by supplying trimethyl gallium (TMG) and ammonia (NH3) onto a sapphire substrate having been heated so as to reach 1020° C. to 1100° C. The raw material liquid 110 is a molten substance prepared by melting metallic gallium and Na as raw materials at a high temperature.
When a crystal is produced in the production apparatus having such a configuration as described above, first in the outside of the production apparatus, the template 201 is disposed in the crucible 107 so as to lie along and to be oriented parallel to the bottom of the crucible 107. Further, metallic gallium and Na as the raw materials are weighed to predetermined amounts and set in the crucible 107.
Then, the crucible 107 is inserted into the air-tight pressure-resistant heat-resistant vessel 103, and the pressure-resistant heat-resistant vessel 103 is set in the growth furnace 100 and connected to the raw material gas feeder 101 through the connecting pipe 114. The growth temperature is set at 850° C. and the nitrogen atmosphere pressure is set at 50 atm (50×1.01325×105 Pa), and while the growth furnace 100 is being swung about the shaft center A, nitrogen gas is dissolved in the Ga/Na molten liquid as the raw material liquid 110 to grow the GaN single crystal on the template 201.
On completion of the growth of the GaN single crystal, the raw material liquid 110 is cooled and solidified in the pressure-resistant heat-resistant vessel 103. Then, the crucible 107 is taken out from the pressure-resistant heat-resistant vessel, the raw material cooled and solidified in the crucible 107 is subjected to a dissolution treatment with ethanol or the like, and the template 201 with the GaN single crystal grown thereon is taken out.